In fuel cells, a fuel and an oxidant are supplied to two electrically-connected electrodes to electrochemically oxidize the fuel, thereby converting chemical energy directly to electrical energy. Unlike thermal power generation, fuel cells are not limited by the Carnot cycle; therefore, they show high energy conversion efficiency. A fuel cell generally comprise a stack of fuel cells, each having an electrolyte layer sandwiched by a pair of electrodes, that is, a membrane-electrode assembly as the basic structure.
Electrochemical reaction at the anode and cathode of fuel cells is developed by introducing a gas such as fuel gas or oxidant gas into a triple phase boundary (three-phase interface) where the gas is in contact with catalyst particles and a polymer electrolyte, the catalyst particles being supported by a carrier (conductor) and the polymer electrolyte ensuring ion conductive paths.
Electrode reaction at the anode side catalyst layer and the cathode side catalyst layer is active when the amount of the catalyst-supporting by carbon particles (e.g., carbon black) is large, resulting in an increase in power generation performance of batteries. However, catalysts used in fuel cells are noble metals such as platinum, and it is problematic in that there is an increase in fuel cell production cost by increasing the supported catalyst amount.
In a reaction electrode in which a catalyst is supported by carbon particles, there is a loss of electrons between the carbon particles and between a separator and the carbon particles, which functions as a current collector. This electron loss is thought to be a cause of stopping an increase in power generation performance.
A fuel cell has been proposed as a prior art for avoiding such problems with production cost and electron loss, in which carbon nanotubes (hereinafter may be referred to as CNTs) are used in fuel cell electrodes. An electrode using CNTs has a small electrical resistance and when compared to the electrode in which a catalyst is supported by carbon particles, there are advantages such that a loss of electrons is inhibited and there is an increase in power generation efficiency. Also, the electrode using CNTs is advantageous in that the supported expensive noble metal catalyst can be efficiently used for electrode reaction.
Because of such advantages, electrode techniques using CNTs have been increasingly developed. For example, a fuel cell technique is disclosed in Patent Literature 1, the fuel cell comprising an electrolyte membrane and a pair of electrodes at both sides of the electrode membrane, wherein at least one of the electrodes is provided with an electroconductive nano columnar body oriented at an angle of 60° or less with respect to the plane direction of the electrolyte membrane, a catalyst supported by the electroconductive nano columnar body, and an electrolyte resin coating the electroconductive nano columnar body.